1. Field of Invention
The present invention relates to amplifier with wide gain range, and more particularly to amplifier with wide gain range, which has low noise level.
2. Description of Related Art
Generally, for a single signal amplifier, its gain value is within a limited range. If it needs more gain value to have higher amplification of signal, usually, several amplifiers are connected in series. FIG. 1 is a drawing, schematically illustrating a conventional circuit of variable gain amplifier. In FIG. 1, the conventional variable gain amplifier is formed by several amplifiers 60, coupled in series. The amplifiers 60 respectively have gain ranges of VGA1, VGA2, . . . , VGN. The variable gain amplifiers having a number of N are controlled by the gain control signal, VC, so as to have the wide gain range.
For the circuit structure in FIG. 1, when it is operated at a low gain, each amplifier 600 is turned on, resulting in high noise level at the low gain.
Another kind of amplifier with wide gain range can be achieved by switching control. FIG. 2 is a drawing, schematically illustrating another conventional circuit of variable gain amplifier. In FIG. 2, several amplifiers 11-14 are connected in series by the switches 21-28 as an amplifier, having a signal input terminal 1 and signal output terminal 2. There are through lines 31-34 between the switches, and coupled with the amplifiers in parallel. The control circuit 3 controls the switches 21-28 via the connection lines 51-54 and modulates the gain values of the amplifiers via the connection lines 41-44. When the operating gain is lower than the maximum gain, just a portion of the amplifiers in the front part is turned on, and the signal is transmitted to the output terminal via the through lines 31-34.
In the structure, for example, just the first-stage amplifier 11 is turned on, the signal transmits through the through lines 32.33.34 connected in series with the action of switches. As a result, the extra loss of signal occurs, and the noise gets worse.
In other words, although the variable gain range can be formed by connecting multiple amplifiers, it causes larger noise level when it is operated at low gain level because multiple amplifiers are connected together. The convention amplifier with wide variable gain range does not further consider the noise level when operating at low gain.